The arts in which for detection of the state of electrically non-conductive fluid radiation from a radiation source ionizes the fluid and the concentration of the ionized fluid is electrically detected are well known. The prior arts are disclosed in U.S. Pat. Nos. 2,611,268, 2,627,543, 2,861,452, 3,683,178 and 3,706,938, for example. Briefly, the prior arts employ variation in ion distribution or electrical conductance across electrodes between which runs the fluid ionized by radiation from a radiation source, said variation produced responsive to the presence, direction, velocity, etc. of the fluid flow. Some of the prior arts include electrode means oppositely disposed laterally of the central plane of the radiation source, so that electrode means may differentially detect the ion distributions or conductances upstream and downstream of the fluid flow. One of the common disadvantages of the prior arts is that the electric signal from electrode means tends to be affected by external induction. More particularly, since a transducer comprising the radiation source and electrode means has a high impedance, an amplifier connected to the transducer is required to have a high input impedance for matching. Due to high input impedance of the amplifier, leads between the transducer and the amplifier tend to have external noise induced. Such external noise causes the output signal from the amplifier to become unstable and also the state of faint fluid flow to be unable to be positively detected. Specially, when the fluid to be measured is liquid radiation passing through liquid has a greater attenuation and also the generated ions have shorter life time, resulting in lower output from the transducer. Thus, the effect by external noise must be avoided. Moreover, due to the floating capacity of the leads between the transducer and the amplifier, the state of the fluid is detectable only with a lower response.
Another disadvantage of the prior arts is that they cannot detect any of the volumetric flow rate, composition and pressure of the fluid. Furthermore, the amplifier is required to be stably operated even when it is placed in varied ambient temperatures.